1. Field of the Invention
This invention relates to a method for the production by efficient dispersion of an inorganic pigment in an aqueous medium, and more particularly to a method for the production of an aqueous dispersion of inorganic pigment, characterized by jointly using, as dispersants, a carboxyl group-containing water-soluble polymer possessing a specific number average molecular weight and/or a water-soluble condensed salt of phosphoric acid and a specific water-soluble anionic modified polyvinyl alcohol.
2. Description of the Prior Art
Calcium carbonate, as an example of inorganic pigments, is inexpensive and excellent in whiteness, affinity for ink, gloss, and printability and, therefore, is counted among the pigments which are popularly used as in the papermaking industry. Further, owing to the dissemination of neutral paper, the amount of calcium carbonate used in the papermaking industry is increasing sharply.
Heretofore, the so-called heavy calcium carbonate possessing an average particle diameter of about 1 micron has been prevalently used in the various grades of calcium carbonate. In recent years, in the total consumption of all grades of calcium carbonate, the proportion of consumption of sedimenting light calcium carbonate having an average particle diameter of 0.1 to 0.5 micron is now increasing in consequence of the growing trend of paper products toward improvement of quality. A decrease in the average particle diameter of such calcium carbonate, however, results in increased difficulty of the dispersion of the compound in an aqueous medium, increased liability of the compound to coagulation, and increased impairment of the stability of the produced aqueous dispersion on aging.
The dispersants heretofore used for calcium carbonate, in the category of inorganic compounds, include such condensed phosphates as pyrophosphates, tripolyphosphates, trimetaphosphates, tetrametaphosphates, and hexametaphosphates, zinc salts, and silicates. These inorganic dispersants, however, entail the disadvantage that the aqueous dispersions produced with the dispersants have no perfect lasting stability. In the case of the dispersion of sedimenting light calcium carbonate in the form of microfine powder, there arises a serious drawback that the film produced suffers from poor water resistance because the dispersion calls for addition of a large amount of dispersant.
As organic dispersants, such polycarboxylates as polyacrylates, polymethacrylates, and polymaleates and polyvinyl alcohol have been known to the art.
Polysodium acrylate, a typical example of polycarboxylates, has been rated rather high in usefulness for the dispersion of heavy calcium carbonate possessing an average particle diameter of about 1 micron. When this dispersant is used in the dispersion of extremely fine calcium carbonate powder having an average particle diameter approximately in the range of 0.1 to 0.5 micron, however, the produced aqueous dispersion has a drawback that it has high viscosity and lacks lasting stability. Polyvinyl alcohol is described in Japanese Patent Laid-Open SHO 60(1985)-262,862, for example, as possessing an ability to provide effective dispersion of inorganic pigments. The dispersant disclosed in Japanese Patent Laid-Open SHO 60(1985)-262,862, however, is insoluble in water and is not sufficiently effective in dispersing calcium carbonate. Further, when this dispersant is used by itself, the produced aqueous dispersion possesses unusually high viscosity. It is, therefore, totally unfit for the production of a highly concentrated aqueous dispersion of calcium carbonate.
For the purpose of eliminating such drawbacks of the conventional dispersants as described above, methods resorting to use of maleic acid copolymers have been proposed by Japanese Patent Publication SHO 54(1979)-36,166, U.S. Pat. No. 4,175,066, Japanese Patent Laid-Open SHO 53(1978)-144,499, and U.S. Pat. Nos. 4,519,920 and 4,555,557, for example. The dispersant disclosed in Japanese Patent Publication SHO 54(1979)-36,166 is rated relatively high because the aqueous dispersion of calcium carbonate obtained therewith exhibits satisfactory flow characteristics (low high-shear viscosity). It nevertheless has much to be desired in the dispersion of the compound in such a high concentration as to produce a solid content exceeding 65% by weight. The dispersant disclosed in U.S. Pat. No. 4,175,066, though capable of producing an aqueous dispersion of microfine calcium carbonate powder with low viscosity, must be used in a large amount and suffers from such drawbacks as high cost and poor resistance of the produced film to water. The dispersant disclosed in Japanese Patent Laid-Open SHO 53(1978)-144,499 is capable of producing an aqueous dispersion of calcium carbonate with such an extremely high concentration as to produce a solid content of 70% by weight. It nevertheless must be used in such a large amount as 1.4% (based on calcium carbonate). The dispersant disclosed in U.S. Pat. Nos. 4,519,920 and 4,555,557 has a serious drawback that the produced aqueous calcium carbonate dispersion exhibits poor flow characteristics (high high-shear viscosity).
Japanese Patent Laid-Open SHO 59(1984)-193,964 proposes a method which resorts to use of an itaconic acid copolymer. The dispersant disclosed therein, however, has much to be desired as to the dispersion of calcite type cubic calcium carbonate possessing an average particle diameter of not more than 0.3 micron.
Aluminum hydroxide, a typical example of inorganic pigments, possesses high whiteness and imparts highly desirable smoothness and gloss to the produced paper and is counted among the pigments popularly used in the papermaking industry. As dispersants for such aluminum hydroxide, Japanese Patent Publication SHO 50(1975)-23,850 proposes use of a copolymer of an .alpha., .beta.- unsaturated carboxylic acid and a hydrophobic vinyl monomer and Japanese Patent Laid-Open SHO 53(1978)-144,498 proposes use of a copolymer of an unsaturated monocarboxylic acid and an unsaturated dicarboxylic acid. These dispersants have much room for further because the aqueous dispersions produced therewith exhibit insufficient flow characteristics.
An object of this invention, therefore, is to provide a novel method for the production of an aqueous dispersion of inorganic pigment.
Another object of this invention is to provide a method for the production of a highly concentrated aqueous dispersion of inorganic pigment possessing low viscosity and high flowability and excelling in lasting stability, with a dispersant used in a small amount relative to the inorganic pigment.